In a display device such as cathode-ray tube display device (CRT), plasma display (PDP), electroluminescence display (ELD) and liquid crystal display (LCD), it is the most cases that film where a transparent layer which is called a hard coat layer is provided on a transparent support is adhered on a display image surface. Generally, a hard coat layer is a layer which is prepared in such a manner that a photopolymerization initiator is compounded with a liquid wherein a monomer having multifunctional group is placed, application and drying are conducted and, after that, hardening is done by irradiation of ionizing radiation such as UV light. Recently, transparent film having an antiglare property for prevention of ghost in the background of display image and for improvement of visual confirmation or transparent film having a preventive function for reflection has been frequently used.
Generally, in a film having an antiglare property, unevenness is intentionally formed on the film surface (the case of a hard coat surface is frequent) for making the light from outside into diffused reflection and, in addition, in a transparent film having a preventive function for reflection, a thin membrane having lower refractive index is formed on the surface of the hard coat layer in order to prevent a lowering of contrast and a ghost of image due to reflection whereby reflection rate is reduced using a principle of optical interference. Transparent film having both functions of antiglare property and prevention of reflection has been also put into the market.
Since the transparent film as such is placed on the outermost surface of the display, users may scrub the front side of the image display with a cleansing instrument or the like whereby scratch is apt to be formed. Therefore, it is important in view of resistance to scratch to enhance the film strength of the hard coat layer and also to make the hard coat layer thick.
However, the hard coat layer which is designed as such has an insufficient softness and flexibility and a bad brittleness and, when it is bent during a handling process of transparent film, cracks may be generated and it has been difficult that both resistance to scratch and brittleness are satisfied.
Incidentally, as one of optical polymerization initiators, active halogen compound has been known and is disclosed, for example, in JP-A-7-311462 and JP-A-9-179299. However, they are the patents where the use is limited to the use as color filters and they do not refer to the formation of hard coat layer as mentioned above.
On the other hand, in a reflection preventive film, it is common that, in order to prevent a lowering of contrast and a ghost of image due to reflection, a film for prevention of reflection is provided on the outermost surface of the display so as to reduce the reflection rate using a principle of optical interference. Therefore, there is a high possibility of scratch and it has been an important problem to give an excellent resistance to scratch.
The film for prevention of reflection as such is usually prepared by forming a low refractive layer having an appropriate film thickness on the outermost surface or, in some cases, by appropriately forming high refractive layer, medium refractive layer, hard coat layer, etc. between the support (base material). In order to achieve a low refractive index, a material having a refractive index of as low as possible is desired for a low refractive layer. Further, since the film for prevention of reflection is used at the outermost surface of the display, a high resistance to scratch is demanded. In order to achieve a high resistance to scratch in a thin membrane where thickness is around 100 nm, strength of the film itself and close adhesion to the lower layer are necessary.
In order to lower the refractive index of a material, there are some means such as introduction of fluorine atoms and lowering of density (introduction of voids) but any of them tend to deteriorate the film strength and close adhesion and to lower the resistance to scratch whereby it is a difficult problem that both low refractive index and high resistance to scratch are satisfied.
In JP-A-11-189621, JP-A-11-228631 and JP-A-2000-313709, there are descriptions for means where a polysiloxane structure is introduced into a fluorine-containing polymer whereby friction coefficient of the film surface is lowered and resistance to scratch is improved. Although such means are effective to some extent for the improvement of resistance to scratch, a sufficient resistance to scratch is not achieved, only by such means, for the film where essential film strength and close adhesion of interface are insufficient.
On the other hand, in JP-A-2002-156508, it is described that hardness increases when an optically hardening resin is hardened at low oxygen concentration. However, a reflection preventing film is efficiently manufactured by webs, there is a limitation for the concentration which is able to be substituted with nitrogen whereby a satisfactory hardness has not been achieved.
In JP-A-11-268240, JP-A-60-090762, JP-A-59-112870, JP-A-4-301456, JP-A-3-067697 and JP-A-2003-300215, there are descriptions for specific means for substitution with nitrogen. However, in order to lower oxygen concentration to such an extent that thin membrane such as low refractive layer is well hardened, there is a problem that large amount of nitrogen is necessary and the cost for manufacture rises.
In JP-B-07-051641, there is a description for a method to wind on a hot roll surface followed by irradiating with ionizing radiation. However, it is also insufficient for hardening until a special thin membrane such as a low refractive layer is well hardened.
In JP-A-55-077742, there is a description for a novel compound for avoiding the fact that conventional active halogen compound is accompanied by sublimation or bad smell. However, such a novel compound is used by combining with an alkali-soluble binder in a system of photoresist.